File and links Settings: Add Sanitization checks - Cant use '.attachments' as attachment folder name

This for sure the same class of problem and there is quiet some “noise” around the general handling of ‘hidden’ directories and files in the forums.

I filed this as a bug because the ‘.attachments’ is accepted without complaining but it doesn’t work. It requires further investigation to learn that dot-directories, which are obviously NOT hidden directories on windows, arn’t processed.

I would expect that if I state a directory explictely it is either rejected as invalid or used.